Neutralization

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In chemistry, neutralization, or neutralisation (see spelling differences) is a chemical reaction whereby an acid and a base react to form water and a salt.

In an aqueous solution, solvated hydrogen ions (hydronium ions, H₃O⁺) react with hydroxide ions (OH^-) formed from the alkali to make two molecules of water. A salt is also formed. In non-aqueous reactions, water is not always formed; however, there is always a donation of protons (see Brønsted-Lowry acid-base theory).

Often, neutralization reactions are exothermic, giving out heat to the surroundings (the enthalpy of neutralization). An example of an endothermic neutralization is the reaction between sodium bicarbonate (baking soda) and any weak acid, for example acetic acid (vinegar).

Common reactions

Most generally, the following occurs:

acid + base → salt + water

For example, the reaction between Hydrochloric acid and sodium hydroxide solutions:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

or as a net ionic equation:

H⁺(aq) + OH⁻(aq) → H₂O(l)

General Formula

• acid + alkali → metal salt + water
  • HCl + NaOH → NaCl + H₂O
• acid + metal → metal salt + hydrogen
  • 2 HCl + Mg → MgCl₂ + H₂
• acid + metal oxide → metal salt + water
  • 2 HCOOH + MgO → Mg(HCOO>)₂ + H₂O
• acid + ammonia → ammonia salt
  • HF + NH₃ → NH₄F

Applications

• Chemical titration methods are used for analyzing acids or bases to determine the unknown concentration. Either a pH meter or a pH indicator which shows the point of neutralization by a distinct color change can be employed. Simple stoichiometric calculations with the known volume of the unknown and the known volume and molarity of the added chemical gives the molarity of the unknown.
• Excess gastric acid in the stomach (acid indigestion) is typically neutralized by the ingestion of sodium bicarbonate (NaHCO₃) or another neutralizing agent such as an antacid.
• Neutralization can also be used to reduce the pain of insect and plant stings. Bee stings can be neutralized with alkalis and wasps with acids. Nettle stings can be neutralized with alkalis like the one found in dock leaves.
• In neutron wastewater treatment, chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. For pH control, popular chemicals include calcium carbonate, calcium oxide, magnesium hydroxide, and sodium bicarbonate. The selection of an appropriate neutralization chemical depends on the particular application.

Calculations

Equal numbers of moles of acid and base are needed for neutralization reactions. Hence, the formula becomes

a × [A] × V_a = b × [B] × V_b

where a is the number of acidic hydrogens and b is the constant that tells you how many H₃O⁺ ions the base can accept. [A] denotes the concentration of acid and [B], the concentration of base. V_a is the volume of acid and V_b is the volume of base.

References

• Metcalf & Eddy. Wastewater Engineering, Treatment and Reuse. 4th ed. New York: McGraw-Hill, 2003. 526-532.